Decreases in soil P availability are associated with soil organic P declines following forest conversion in subtropical China

•Converting natural forest to plantations reduced soil P availability.•Converting natural forest to plantations decreased soil microbial biomass.•Labile and moderately labile organic P were the potential source of available P.•Phosphatase activity and labile Pi were positively related. The large-scale conversion of natural forests to plantations profoundly alters the accumulation and transformation of soil nutrients. However, our understanding of how soil P responds to forest conversion is far from complete. This study aims to explore the changes in various soil P fractions associated with forest conversion in subtropical forest soils, where P is known to be relatively scarce. Soil samples were collected from an evergreen broad-leaved forest, a secondary forest, and two plantation forests. The soil properties, P fractions, amino sugars, and phosphatase activities were determined. The replacement of natural forests by plantation forests was associated with reductions in soil labile inorganic P (Pi) and organic P (Po), moderately labile Po, and total P. Soil amino sugars, which are indicative of historical populations of soil microbes, consistently decreased following forest conversion. We also found significant positive relationships between most P fractions and amino sugars content. A structural equation model suggested that Po, especially labile and moderately labile Po, were the potential sources of labile Pi. The positive relationship between labile Pi and acid phosphatase activity indicates that microbes are involved in soil P transformations by regulating P-hydrolytic enzymes activity. Overall, our work suggests that forest conversion significantly decreased most soil P fractions, including labile Pi, labile Po, and moderately labile Po, partly due to the declines in microbial populations that play an important role in releasing P from litter and root necromass. Organic P may play a vital role in sustaining P availability through organic P decomposition facilitated by microbial phosphatases, resulting in the release of labile P in the forest soils. These findings provide a mechanistic understanding of soil P cycling following forest conversion in low available P subtropical forests.